1. Field of the Invention
This invention relates to non-ionic, hydrophilically-modified, water-dispersible polyisocyanate mixtures containing (cyclo)aliphatically bound isocyanate groups, to a process for their preparation and to their use as cross-linking agents in two-component aqueous compositions.
2. Description of the Prior Art
There has been a great increase in recent years in the importance of aqueous lacquers and coatings due to the increasingly stricter guidelines concerning emission of the solvents released during the application of coatings.
Although aqueous coating compositions have become available for many fields of application, in many cases they can only achieve the high quality of conventional solvent-containing coating compositions with regard to the resistance to solvents and chemicals and mechanical load bearing capacity if a chemical cross-linking reaction is carried out during formation of the films.
Attempts to carry out the cross-linking of aqueous lacquers with compounds containing free isocyanate groups (i.e., to apply the concept of polyurethane reactive lacquers commonly used in the field of solvent-containing coatings to aqueous systems) have only rarely been carried out.
DE-OS 2,708,442 describes the addition of monomeric organic diisocyanates for improving the properties of aqueous polyurethane dispersions. According to DE-OS 3,529,249, organic polyisocyanates may be used for increasing the resistance to solvents and chemicals and improving the wearing properties of coatings based on water-dispersible homo- and copolymers. The described positive effects on the properties of the coating compositions may be attributed to a formation of a "urea shell" around the dispersed polymer particles resulting from the reaction of the isocyanates with water. The polyisocyanates used as additives thus do not function as cross-linking agents for synthetic resins or synthetic resin precursors containing isocyanate reactive groups which are dispersed in water.
The preparation of an aqueous polyurethane reactive system from selected aqueous polyhydroxyl compounds which have an emulsifying action and low viscosity free polyisocyanates is described in DE-OS 3,829,587. This process results in coatings whose properties are equal to those of typical solvent-based two-component coating compositions.
Since organic polyisocyanates are generally difficult to emulsify in water, the process of DE-OS 3,829,587 is limited to specific polyol dispersions which have an emulsifying action. The application of this process to any aqueous binders containing isocyanate reactive groups would require the use of hydrophilically modified, self-emulsifiable polyisocyanate components.
British Patent Specifications 1,417,618 and 1,417,276 describe reaction products of organic polyisocyanates, in particular aromatic polyisocyanates, with i) polyethylene glycol monoalkyl ethers having at least 5, but in the examples always more than 10, ethylene oxide units or ii) diesters of malonic acid and polyethers of this type as surface-active substances. According to GB-PS 1,444,933 and DE-OS 2,908,844 stable aqueous emulsions of aromatic polyisocyanates are prepared.
Aqueous isocyanate emulsions obtained using emulsifiers such as those prepared by the reaction of di- or polyisocyanates with special monofunctional polyethylene oxide polyethers having at least 10 ethylene oxide units are described in DE-OS 2,921,681.
Aromatic polyisocyanates which are hydrophilically modified with alkylene oxide polyethers are described in EP-A-61,628 and EP-A-95,594.
In the form of aqueous emulsions, such products are preferably used as adhesives or binders, for example, for gluing chipboard, for the production of moulded articles or for gluing polyurethane foams (DE-OS 2,447,135, 2,610,552 and 2,921,726 and GB 1,502,777, 2,075,038 and EP-A-0310-345).
Aromatic polyisocyanates are, however, limited in their usefulness as cross-linking components for aqueous coating compositions systems due to their high reactivity with water and the consequent evolution of carbon dioxide and the well known lack of resistance to yellowing of coatings obtained from aromatic polyisocyanates.
Isocyanate functional cross-linking agents for high quality, light-fast, aqueous coating compositions can only be obtained from less reactive (cyclo)aliphatic polyisocyanates.
Water dispersible compositions containing such (cyclo)aliphatic polyisocyanates are disclosed in EP-A-206,059 and contain, as emulsifiers, reaction products of polyisocyanates with monohydric or polyhydric polyalkylene oxide alcohols consisting of at least one polyether chain having at least 10 ethylene oxide units. However, only polyether alcohols having more than 15 ethylene oxide units per molecule have been used in the Examples. The water-dispersible compositions disclosed in the prior publication are used as additives for aqueous adhesives.
The polyisocyanate compositions described in this prior publication are, however, not very suitable as crosslinking agents for aqueous solutions or dispersions of binders or binder components containing isocyanate reactive groups, in particular alcoholic hydroxyl groups.
In two-component coating compositions, a basic requirement for good lacquer technical properties is a very homogeneous distribution of the cross-linking component in the binder. In aqueous polyurethane reactive systems, this requires the use of polyisocyanates which have been rendered highly hydrophilic and can be emulsified in exceptionally fine form. The water-dispersible polyisocyanate preparations described in EPA 206 059 frequently tend to crystallize when stored at room temperature if they contain the relatively large quantities of emulsifier required for obtaining good emulsification.
Emulsification in water of polyisocyanates which have been rendered hydrophilic is generally associated with considerable changes in viscosity. The viscosity of the water-in-oil emulsion originally formed progressively increases with increasing absorption of water until finally phase reversal results in the formation of an oil-in-water emulsion and a decrease in viscosity. The viscosity maximum which is required to be overcome is so high, in particular for the emulsification of the polyisocyanate compositions EP-A-206,059 which have a high emulsifier content, that in many cases these compositions can only be stirred into water and distributed sufficiently finely if high shearing forces are employed (e.g., high speed stirrers) or considerable quantities of organic co-solvents are added. The presence of organic co-solvents in aqueous coating compositions should be avoided to the extent possible for ecological reasons and the use of high speed stirrers in most cases causes troublesome foaming of the coating compositions.
Accordingly, it is an object of the present invention to provide hydrophilically modified polyisocyanate mixtures containing (cyclo)aliphatically bound isocyanate groups which do not have the above-described disadvantages of the known water-dispersible polyisocyanates. It is an additional object of the present to prepare these hydrophilically modified polyisocyanate mixtures from known polyisocyanates which are suitable as starting components for the production of polyurethane resins, preferably as cross-linking agents for aqueous binders or binder components in aqueous coating compositions. These polyisocyanates include in particular those based on derivatives of 1,6-diisocyanatohexane (HDI) or 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethyl-cyclohexane (IPDI) which contain uretdione, isocyanurate, urethane, allophanate, bioret and/or oxadiazine groups.
These objects may be achieved with the polyisocyanate mixtures according to the present invention which are described hereinafter and the process for their preparation. The present invention is based on the surprising discovery that (cyclo)aliphatic polyisocyanates which have been hydrophilically modified by a reaction with monofunctional polyether alcohols containing a statistical average of less than 10 ethylene oxide units can very easily be emulsified in water without troublesome fluctuations in viscosity.
In accordance with the process according to the invention highly hydrophilic polyisocyanate mixtures can be obtained in very finely emulsified form. In contrast to the known water-dispersible polyisocyanate compositions, those according to the invention show no tendency to crystallize even when they have the same ethylene oxide unit content.
Even though
1) polyethylene oxide polyethers having a minimum chain length of 5 ethylene oxide units are disclosed in some of the previously discussed publications, for example, in GB 1,444,933 and DE-OS 2,447,135, 2,908,844 and 2,921,726, as suitable for the preparation of emulsifiers, PA0 2) monomeric (cyclo)aliphatic diisocyanates such as HDI and IPDI are included in long lists of suitable polyisocyanates and PA0 3) DE-OS 2,921,726 also discloses modified polyisocyanates, for example, polyisocyanates modified with isocyanurate or biuret groups, PA0 a) have an average isocyanate functionality of 1.8 to 4.2, PA0 b) contain 12.0 to 21.5% by weight of (cyclo)aliphatically bound isocyanate groups (calculated as NCO, molecular weight=42) and PA0 c) contain 2 to 20% by weight of ethylene oxide units (calculated as C.sub.2 H.sub.4 O molecular weight=44) arranged within polyether chains, the polyether chains containing an average of 5.0 to 9.9 ethylene oxide units. PA0 A) a polyisocyanate component having an average isocyanate functionality of 2.1 to 4.4 and containing one or more polyisocyanates in which all the isocyanate groups are (cyclo)aliphatically bound with PA0 B) a monovalent polyalkylene oxide polyether alcohol containing an average of 5.0 to 9.9 ethylene oxide units. PA0 a) have an average isocyanate functionality of 1.8 to 4.2, preferably 2.0 to 4.0, PA0 b) have an isocyanate content of 12.0 to 21.5% by weight, preferably 15.0 to 20.5% by weight and PA0 c) contain 2 to 20% by weight, preferably 3 to 17% by weight, of ethylene oxide units arranged within polyether chains. PA0 (i) polyisocyanates which have been hydrophilically modified according to the invention and PA0 (ii) polyisocyanates which have not been hydrophilically modified of the type exemplified.
it would not be possible for a skilled artisan to predict based on these publications that (cyclo)aliphatic polyisocyanates, in particular modified polyisocyanates, could very easily be stirred into water to form stable, finely divided emulsions after they have been reacted with polyether alcohols containing less than 10 ethylene oxide units, and that this could be achieved without the necessity of overcoming a noticeable viscosity maximum. These findings are surprising since according to the teachings of EPA 206,059 it was to be expected that aqueous emulsions of such polyisocyanates could only be prepared with the aid of emulsifiers containing at least 10 ethylene oxide units, and according to the Examples, such polyisocyanates could only be obtained with the aid of emulsifiers having at least 15 ethylene oxide units.